The present invention relates to a nitride semiconductor laser characterized by having a stress concentration suppressing layer between an active layer and a cap layer.
A GaN-based group III–V compound semiconductor (hereafter, referred to as a GaN-based semiconductor), which is a direct transition semiconductor whose forbidden band gap is in a range from 1.9 eV to 6.2 eV, enables a realization of a semiconductor light emitting device, such as a semiconductor laser diode (LD), a light emitting diode (LED) and the like, in which a light emission can be obtained from a visible region to a ultraviolet region. Thus, in recent years, development in the field has been vigorously advanced. Among them, actual usage of a blue-violet semiconductor LD from which a light having a light emission wavelength of about 400 nm is obtained is especially required in order to improve a recording density of an optical disc or the like, in a field of an optical recording. Also, a blue semiconductor LD having a light emission wavelength of about 460 nm is expected to be applied to a laser display. Moreover, an ultraviolet semiconductor LD having a light emission wavelength of 380 nm or less is expected to be applied to a light source for phosphor excitation.
Those GaN-based semiconductor light emitting devices are typically provided with GaN-based semiconductors grown on a substrate. Conventionally, as the substrate on which this GaN-based semiconductor is grown, there is no proper substrate having an excellent lattice matching property with GaN. Thus, a sapphire substrate is mainly used. However, a lattice mismatching with the GaN and a thermal expansion coefficient difference from it are very large. In this way, when the lattice matching with the substrate is poor and the thermal expansion coefficient difference from the substrate is large, influence on a crystallinity of a GaN-based semiconductor layer grown on the substrate is severe. Hence, a large quantity of dislocations, such as on an order of 108 to 1010/cm2, is implanted into the GaN-based semiconductor layer in order to relax that distortion. Among them, a threading dislocation especially transmitted in a thickness direction of a film is harmful for an active layer of a device formed near a film surface, and it acts as a current leakage portion and a non-light-emission center. So, threading dislocation is known to damage electrical and optical properties of the device.
Thus, in order to manufacture the GaN-based semiconductor device, threading dislocation must be reduced as much as possible. In recent years, as a method of reducing threading dislocation, an epitaxial growth method has been employed which uses a lateral direction growth, which is represented by an ELO (Epitaxial Lateral Overgrowth) method. The present inventors have employed the ELO method and tried to reduce a dislocation density of a GaN epitaxial film. At that time, the dislocation density within a Wing portion (a portion resulting from the lateral direction growth) under an optimized condition could be reduced to an order of about 106/cm2 or less. As a result, in the above-described GaN-based semiconductor laser manufactured by the present inventors, it was evident that a device life was improved by such as about 200 hours under conditions of 50° C. and 30 mW.
However, even in the case of the above-mentioned GaN-based semiconductor laser, it is difficult to say that those result can be sufficiently attained in actual usage. There is a margin for further improvement so as to improve the device life.
A subject of the present invention is to provide a nitride semiconductor laser, in which a cleaved end surface is flat, and a breakdown of a laser end surface induced during operation can be suppressed, resulting in a long life.